I need a new Video card

I just fried my GeForce 9280! My motherboard is an Asus P4S8X with Sis648 chipset.Does this mean I should use an AGP card? Can I use a PCI-express card? I am currently leaning to the Ati X850 pro 256mb AGP or the GeForce 7800 GT OC 256mb.
The burning questions are:Should I stick with the AGP cards?
Will my board even run a PCI-Express?
Is there a better card I should be looking at.
I am a heavy game player!! I need a high fps--Lagging flips me out!!!
Any advice will be extremely appreciated.
Also I need to Know if my power supply and cpu are adequate for a more powerful board.I have a 2.4 and max of 12v.

Cool--As soon as it stops snowing--off to the store!The 7800 is going for $400, the X850 is $300. Thanx 4 the info!!
Change in plan--my PSU is only 300w-won't handle the 7800-barely makes spec for the X850!
Is theRadeon 9600pro/td any good?